1. Field of the Invention
The present invention relates to an electric fastener which fastens bolts to a member to be fastened such as steel frames.
2. Description of the Related Art
FIGS. 6 to 9 illustrate the steps of fastening a plurality of steel frames 3 with a one-side bolt 2 having a shear washer. FIG. 11 illustrates a waveform of a value of an electric current flowing in a driving motor 15 of an electric fastener 1 for fastening the one-side bolt 2 (hereinafter, simply “current value”) at the time of the fastening steps.
In FIG. 6, a side from which the one-side bolt 2 is inserted into the steel frames 3 is designated by A and the opposite side is designated by B. As publicly known, the one-side bolt 2 can fasten a nut on the insertion side A differently from normal bolt and nut. That is to say, the bolt and nut can be fastened from one side.
As shown in FIG. 6, the one-side bolt 2 having shear washer is constituted such that a bolt head 22, a tube member for collar 27, a spacer tube 29 and a shear washer 25 are fitted into a shear bolt 21 in this order so as to be slidable in an axial direction, whereupon a nut 24 is screwed thereinto.
The shear bolt 21 has a shearing chip 23 at its front end. The shear washer 25 touches an end surface of the spacer tube 29, and has a flange 26 which is sheared by pressurizing the spacer tube 29 in the direction shown by an arrow C so as to project inwardly.
In the electric fastener 1, a casing 11 houses a planetary gear reduction mechanism 14 having one input shaft and two output shafts. An outer socket 12 and an inner socket 13 which concentrically protrude from the front end of the casing are connected to the two output shafts of the planetary gear reduction mechanism 14 so as to be capable of rotating in opposite directions. A motor 15 is connected to the input shaft of the planetary gear reduction mechanism 14 (for example, see Japanese Patent Application Laid-Open No. HEI09-314478).
As shown in FIG. 6, the one-side bolt 2 is inserted into holes 31, which are opened on a plurality of superposed steel frames 3 in advance, on the insertion side A, and the tube member for collar 27 is protruded from the opposite side B.
The inner socket 13 of the electric fastener 1 is fitted into a bolt chip 23, and the outer socket 12 is fitted into the nut 24. A trigger 16 of the electric fastener 1 is pulled so that a motor starting switch 47 is turned ON.
After the starting switch 47 is turned ON and the motor 15 is electrically connected, instantly the current value of the motor 15 abruptly rises as shown by “E” in FIG. 11, and immediately after that, the value abruptly falls as shown by “F”. This is a so-called rush current phenomenon.
At this stage, a load is hardly applied to the motor 15, and the current value converges to a lower value.
As shown in FIG. 7, the bolt 21 is drawn to the side of the electric fastener by the rotation of the nut 24. An inward flange 26 of the washer 25 abuts against the end surface of the spacer tube 29 (hereinafter, this state is called “seating of the nut”) in a state where no gap is present between the end surface of the tube member for collar 27 and the bolt head 22 and between the tube member for collar 27 and the spacer tube 29.
When the nut 24 is seated, a thrust force produced by the rotation of the nut is applied as an axis-direction compressing force to the tube member for collar 27 and the spacer tube 29. The spacer tube 29 fitted and restrained in the holes 31 of the steel frames 3 cannot be deformed. Therefore, the tube member for collar 27 positioned on the outside of the holes 31 is plastically deformed by the axis-direction compressing force so as to expand into a collar shape (see FIG. 7).
When the screw thrust force produced by the rotation of the nut 24 is applied, the spacer tube 29 pushes and shears the inward flange 26 of the shear washer 25 (see FIG. 8) (hereinafter, this state is called “shearing of the shear washer 25”).
During the fastening operation, the inner socket 13 engaged with the bolt chip 23 is a reactive force receiver, and prevents the nut 24 and the bolt 21 from rotating together.
In FIG. 11, “G” indicates the current value at the time when the nut is seated, and “H” indicates the current value at the time when the shear washer 25 is sheared. It is found that the current value gradually increases from “G” to “H”.
At the moment when the inward flange 26 is sheared, the current value abruptly falls and converges to a low value.
As shown in FIG. 8, the inward flange 26 is sheared and is allowed to go into the washer 25 of the spacer tube 29. As a result, a collar portion 28 of the tube member for collar 27 is seated in the steel frame 3.
The above steps are the fastening steps of the bolt and nut in order to form the collar portion 28 on the tube member for collar 27 and to seat the collar portion 28 in the steel frame 3, and thus the fastening force is not applied to the steel frames 3.
After the collar portion 28 is seated in the steel frame 3, the fastening to the steel frames 3 is started, and when the nut 24 is fastened up completely, the rotation of the outer socket 12 is stopped. The inner socket 13 rotates in a direction opposite to the outer socket 12 so as to shear the bolt chip 23.
When the bolt chip 23 is sheared, it is ensured that the one-side bolt 2 is fastened by a predetermined torque.
In FIG. 11, “J” indicates the current value at the time of starting the fastening to the steel frames and “O” indicates the current value at the time of shearing the bolt chip 23. Since the load to the motor 15 instantly falls at the moment when the bolt chip 23 is sheared, the current value abruptly falls.
A plurality of bolts are used for the fastening to the steel frames. When the bolts are fastened up one by one until the bolt chip is sheared, a problem such that the bolts cannot be uniformly fastened arises. Therefore, in a normal manner, the bolts 21 should be primarily fastened by a constant torque weaker than the torque for shearing the bolt chip, and then should be finally fastened until the front end chip 23 is sheared.
Conventionally, a fastener for primary fastening only which automatically stops when a primary fastening torque is attained and a fastener for final fastening only which fastens the bolts until the bolt chip 23 is sheared should be used properly. This causes inconvenience and high cost in the screw fastening.
Also in the normal bolt/nut fastening, when the threads of the nuts and/or bolts are damaged, a current pattern similar to that at “E” to “I” in FIG. 11 appears. That is to say, when the threads of the nuts and/or bolts are normal, the current value exceeds “H” in FIG. 11 and continues to rise. The current value, however, occasionally falls abruptly after “H”. This is because at the stages of fastening the nuts and/or bolts, the fastening torque is lost and the load of the motor sharply reduces. It can be, therefore, determined that the threads of the nuts and/or bolts have some sort of defect. In this case, it is necessary to send an alert to a worker of the fastening.
In order to solve the above problems, the present invention discloses an electric faster which performs a suitable control in the follow manner. In the case of the one-side bolts having shear washer, the shearing of the shear washer is detected on the basis of the pattern of the current value at a process of fastening the bolts and nuts. Further, in the case of the normal bolts and nuts, faulty fastening due to a defect or the like of the threads during the fastening is detected on the basis of the pattern of the current value. In the former case, when the current value rises to a value corresponding to the primary fastening torque after the shear washer is sheared, the electric fastener is automatically stopped. In the latter case, when the faulty fastening is detected, an alert is sent immediately.
Further, the present invention discloses an electric fastener which solves the following problem and can give a correct control command. The abrupt rise in the current value even with no load at the moment when the motor of the electric fastener is switched ON cannot be discriminated from the current value at the time of shearing the shear washer or the current value at the time of attaining the primary fastening torque.